1. Field of the Invention
The present invention relates to an automatically retractable utility knife, and more particularly to an automatically retractable utility knife in which the blade carrier is integrally molded with the retraction spring, in which the blade carrier and housing are adapted to be used by both right-handed and left-handed persons, and in which the blade housing is adapted to receive either automatically retractable or indexed blade carriers. More generally, the invention relates to a plastic spring that may be utilized in any type of device in which the spring is only momentarily contracted or lengthened.
2. Description of the Related Art
Utility knives are well known in the art, such as those shown in U.S. Pat. Nos. 5,121,544; 5,025,558; and 4,621,425. Utility knives are typically constructed of a hollow housing having a slidable internal razor blade carrier on which a razor blade is mounted. A thumb piece is connected to the blade carrier and extends exteriorly of the housing through a slot in the housing. Using the thumb piece, the blade may be extended out of the housing through a slot at a distal end of the housing, and retracted into the housing. Most utility knives are indexed, i.e., the blade may be moved between one of several latched positions, including fully retracted and fully extended positions.
Another type of well-known utility knife is the automatically retractable type, in which the blade is normally retracted in the housing and requires continuous application of pressure by the user on the thumb piece for the blade to remain in the projected position. A spring attached between the housing and the blade carrier automatically retracts the blade upon release of the external thumb mechanism. The spring tension is such that when the blade is projected and is inserted into a material being cut, e.g., cardboard, the friction between the blade and the material is sufficient to retain the blade in the projected position. Automatically retractable utility knives are shown, for example, in U.S. Pat. Nos. 4,139,939 and 5,012,581. These types of utility knifes are expensive to manufacture due to the cost of the spring and are difficult to assemble because the spring must be secured between the blade carrier and housing. Also, the housings of conventional automatically retractable utility knives are not adapted to receive indexed blade carriers, thereby necessitating the use of separate molds for indexed knives and automatically retractable knives.
Another shortcoming of conventional utility knives is that they are typically designed to be used by right-handed persons or left-handed persons, but not by both. In those knives that are not "handed", i.e., may be used by either right-handed or left-handed persons, the blade must be reversed in order to change whether the knife is right or left handed, at great inconvenience to the user.
In most retractable utility knives, the housing is constructed of two halves which are locked together by means of a screw located toward the center of the housing. In order to replace the blade, it is necessary to unscrew the halves of the housing, which requires a screwdriver, and which is both time consuming and inconvenient. To overcome this shortcoming, utility knives with plastic housings have been developed which incorporate locking mechanisms that may be released without tools. For example, U.S. Pat. No. 5,121,544 (embodied in the Stanley.TM. Model 10-165 retractable utility knife) relates to a retractable utility knife that is constructed of separable plastic halves and includes a rear snap-locking mechanism. In order to operate the locking mechanism, the user must push an actuating lever and slide the halves in opposite directions generally parallel to a central axis of the housing, i.e., generally parallel to the direction of movement of the blade, before they release from each other. This locking mechanism is difficult to operate.
Prior art devices that utilize springs to return a movable member to its starting position and/or to provide shock absorption are also well know in the art and are too numerous to discuss in detail. Such devices typically utilize a metal spring that is constructed of a resilient material, usually a tempered spring steel, that is extremely hard and has enough memory to return to its original shape when subject to compression, tension, flexing, or other types of motion. Most metal springs are coiled of hard drawn music wire. Some metal springs, such as leaf or spiral springs, are constructed of flat rolled material.
Although metal springs have excellent memory, they are difficult and expensive to produce. Most coil or spiral-type springs must be coiled, cut, and squared or looped at each end, thereby requiring four operations. Higher quality springs must be made in an annealed state and post-heat-treated for extra hardness. In order to prevent rust, most metal springs must also be plated. Finally, many springs must be prelubricated with wax or grease to reduce wear. Because of the number of steps and the materials involved in manufacturing metal springs, these springs tend to be relatively expensive, and generally may not be constructed integrally with the part that the spring is moving.
Hughes, U.S. Pat. No. 5,164,871 relates to a magnetic disk cassette which includes a case formed of a molded plastic material defining an interior space in which the disk is located. A shutter formed of a plastic material is reciprocally movable on the case between an open position, in which the disk may be accessed through a window in the case, and a closed position in which the shutter covers the window. An elongate spring is integrally molded with either the case or the shutter so as to exert a bias force on the shutter toward the closed position.
One shortcoming of the Hughes device is that the plastic spring tends to set when left in a compressed position. Accordingly, if the Hughes disk is left in a disk drive with the shutter open for an extended period of time, the spring becomes set in the compressed position. As a result, when the disk is removed from the disk drive, the shutter will not entirely close.
Accordingly, it would be desirable to have a plastic spring that may be used to replace metal springs in momentary applications, and that may be integrally molded with the member that the spring is used to bias.
Accordingly, it would be desirable to have a utility knife housing which is adapted to receive automatically retractable or indexed blade carriers so that the utility knife housing may be used for either application.
It would also be desirable to have an automatically retractable utility knife in which the blade carrier is integral with the retraction spring so as to reduce the cost and complexity of manufacture.
It would be further desirable to have a utility knife that does not require the removal of the blade in order to be used by right or left handed persons.
Finally, it would be desirable to have a utility knife with a hand-operated release mechanism that is inexpensive to manufacture and simple to use.